Recent advancements in laser technology have enabled ophthalmologists to perform many surgical procedures on the eyes in their own offices on an outpatient basis rather than employing the facilities of a hospital or clinic. These procedures are performed with the patient in a sitting or reclined position in a specially designed ophthalmological chair. In order to properly adjust the position of the patient relative to the equipment used in the surgery, the chair must be movable both vertically and rotationally with a minimum of effort.
Patient chairs for ophthalmologists or other physicians with similar requirements generally comprise a chair portion including a seat support and back rest which are mounted above the floor upon a lift base. In the prior art, lift bases for ophthalmological chairs and the like include a cylinder having a piston which is vertically movable for raising and lowering the seat support and back rest of the chair. In some designs, the seat support portion of the chair is mounted by bearings to the piston to permit rotation of the chair with respect to the piston. The piston is rotatably fixed, but movable vertically relative to the cylinder to raise and lower the seat and back support. A locking mechanism such as a brake is provided to prevent rotation of the seat support and back rest relative to the piston as desired.
The bearing connection between the piston and seat of the prior art lift bases described above is relatively expensive and has been eliminated in other prior art lift base designs. In these prior art designs, the piston is fixed to the seat support of the chair and is both vertically movable and rotatable relative to the cylinder.
The problem with prior art lift base designs having a rotatable piston is that the chair cannot be locked against rotational movement without also preventing vertical movement of the piston relative to the cylinder. In some surgical procedures, it is desirable to secure the seat from rotational movement while allowing the seat to continue to be raised and lowered. But in prior art lift bases employing a rotatable piston, the locking mechanism which prevents rotation of the seat and piston must be unlocked before the piston can be raised or lowered to adjust the vertical position of the seat.